Multiple "et al" citations with same first author but different co-authors -- how to combine them

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Thanks for your reply. I do have the Letter Format as you describe, but the output in my document is:

Smith et al. (2000a), Smith et al. (2000b)

I assume the result is not Smith et al. (2000a,b) because the “et al” authors are different in each reference.

I am using X6 and Word 2013